1. Field of the Invention
The invention relates to a flex cable for interconnecting together a plurality of printed circuit boards through a connector assembly, and in particular, to a split flex cable that is coupled to the connector assembly, thereby providing an increased electrical performance and enhanced mechanical flexibility.
2. Background Information
Cables typically are coupled with an electronic connector assembly at opposing ends that are matable with a printed circuit board of electronic devices. Such a cable connector assembly is well known in the connector assembly art and includes a universal serial bus (USB)-type connector assembly, a parallel connector assembly, a serial connector assembly, a VHDM (very high density metric) connector assembly, and the like. The VHDM connector assembly may be used for a high-density connection between electronic devices.
In computer systems, for example, flex cables having connector assemblies at each end are utilized for a high-speed interconnection between a first circuit board assembly and a second circuit board assembly. However, when a single flex cable interconnects more than two circuit boards, the end of the flex cable should be understood to mean each portion of the flex cable that is connected to each printed circuit board through the VHDM connector assembly.
FIGS. 1 illustrates a conventional flexible or flex cable incorporated with VHDM connector assemblies for interconnecting two circuit boards. A flex cable 10 and two connector assemblies 12a and 12b (hereafter may be referred to as 12 for simplicity) jointly interconnect together a first circuit board 14 and a second circuit board 16. Each of the circuit boards 14 and 16 may be a portion of a separate computer system. The connector assembly 12, for example a VHDM connector assembly, is electrically and mechanically coupled to connecting portions of the flex cable 10 through vias, such as by soldering or the like.
FIG. 2 is a schematic plan view of a conventional VHDM connector assembly. The VHDM connector assembly 12 has signal pins that are schematically shown from a plan view of the VHDM connector assembly 12 and labeled by a row designation (A-H) and a column designation (1-N). These signal pins are connected to the flex cable 10 and also received in sockets (not shown) of corresponding mating connectors (not shown) on the circuit boards 14 and 16. The connector assembly 12 has a limited number of signal pins arranged along a width direction W of the connector assembly 12, and thus has a predetermined number of rows (e.g., eight rows in FIG. 2). In contrast, the VHDM connector assembly 12 may have, in its length direction L, as many columns of signal pins as required.
Each column of signal pins is separated from adjacent columns of signal pins by ground shield GS. Each ground shields GS has a predetermined number of ground shield solder tails or ground pins (not shown) that are arranged along the width direction W of the connector assembly 12. The connector assembly with eight-row signal pins conventionally has seven rows of ground pins shown as dotted circles in FIG. 2.
The signal pins and ground pins of the connector are inserted into corresponding vias of the flex cable that are arranged in a pattern that the pins of the VHDM connector assembly 12 can be inserted into to the vias.
Each connecting portion 110 and 120 of the flex cable 10 has a plurality of signal vias and a plurality of ground vias. For example, as shown in FIG. 3, the connecting portion 110 has eight signal vias A-H in each signal column and seven ground vias J-P in each ground column, so as to correspond to the pins of the VHDM connector assembly 12. The signal column and ground column are alternately disposed and respectively extend along a width direction of the flex cable 10.
Conventionally, a VHDM connector assembly is connected to only one single connecting portion of a flex cable that extends and integrally formed in its length direction L. FIG. 4 shows the flex cable 10 and the VHDM connector assemblies 12 of FIG. 1 with the flex cable 10 and connector assemblies being connected to each other. The VHDM connector assembly 12a is coupled only to a first connecting portion 110 of the flex cable 10, and the VHDM connector assembly 12b is coupled only to a second connecting portion 120 of the flex cable 10. Each connecting portion 110 and 120 is integrally formed in its length direction. Accordingly, the single flex cable 10 interconnects the two circuit boards 14 and 16 through the connector assemblies 12a and 12b. 
In case of a complex system, more pins are required in the VHDM connector assembly, and a large number of vias and a large length of signal wirings are required in the flex cable, resulting a significant increase in the width of the flex cable or in the number of layers on a flex cable.
For example, if four busses B1 to B4 of signal lines are required to interconnect the two circuit boards 14 and 16 as shown in FIG. 4, these four bus lines B1 to B4 should be included in the single flex cable 10. Each buses may be comprised of a plurality of line, e.g., 16 lines. The number of bus lines that could run on one signal layer of the flex cable 10 varies depending on the number of signals in a bus line and the allowable overall width of cable. The number of lines that could run on a single layer also depends on a width of each line and a line-to-line spacing, both of which have signal integrity and manufacturability limit For example, so as to dispose all of the busses B1 to B4 on a single layer, a width of the cable WC1 should be increased accordingly. However, such an increased width WC1 may exceed the allowable overall width of the flex cable, increase the manufacturing cost, and reduce the flexibility of the flex cable.
Alternatively, the bus lines B1 to B4 of the flex cable 10 may be disposed on two or more separate signal layers of the flex cable based on the signal amounts and the constraint for the overall width of cable. However, this configuration also requires increased number of layers in a single flex cable. A higher number of layers on flex cables increase cost, negatively affect manufacturability and reliability, and result in more stiff cables that do not meet flexibility or other mechanical packaging requirements.
FIG. 5 illustrates required number of layers for interconnecting a four-drawer system. A single flex cable 50 connects drawer #1 to drawers #3 and #4, and drawer #2 to drawers #3 and #4. The single flex cable 50 includes buses of lines B51 to B54. Each of lines B51 to B54 represent a bus that interconnects drawers #1 and #3, drawers #1 and #4, drawers #2 and #3, and drawers #2 and #4. However, so as to dispose those four bus lines on a single signal layer, the bus B54 necessarily cross the buses B51 and B52, thereby making it impossible to use a cable with a single layer. At least two layers are required in the flex cable 50 for interconnecting the four-drawer system as shown in FIG. 5. This increased number of layers reduces the flexibility of the flex cable. Further, when the flex cable 50 is configured to have an additional layer for the bus B54, the bus B54 necessarily crossover the remaining layer or layers on which other bus or buses are disposed. Thus, the flex cable 50 should have an additional space for such crossovers, resulting an associated bad SI impact on the flex cable 50.
A flex cables that is split along a length direction, e.g., designated by an arrow L in FIGS. 2 and 3, of the VHDM connector assembly has been developed. See “Modified VHDM connectors and modular flexible circuitry allowing for increased design flexibility and manufacturability”.
However, when the connecting portion of the flex cable is split in its length direction that is perpendicular to its width direction, each split part of the connecting portion of the flex cable has the same length in the length direction of the connector assembly to which each split part is connected. Thus, each split part should be formed as an integrated element that has the same length as an unsplit connecting portion of the flex cable, thereby limiting design flexibility.
In addition, the conventional VHDM connector assembly 12 is, e.g., on a 2.0×2.25 mm pitch. Accordingly, the vias of the corresponding flex cable 10 has a pitch PL (see FIG. 3) of 2.0 mm in the length direction and a pitch PW (see FIG. 3) of 2.5 mm in the width direction. Accordingly, when the connecting portion of the flex cable 10 is split in the length direction L that is perpendicular to the width direction W, the flex cable 10 is split across the 2.5 mm pitch which is wider than the 2.0 mm pitch, thereby limiting the flexibility of the flex cable.
Moreover, such a flex cable split in its length direction does not give any solution to the layer number increase in the flex cable for connecting a multi-drawer system.